1. Field of Invention:
This invention relates generally to electromagnetic flowmeters, and more particularly to a meter of this type in which a noise voltage generated in the primary of the meter and offset voltages produced in amplifiers included in the secondary and compensated for to provide an accurate flow rate reading.
2. Status of Prior Art:
In an electromagnetic flowmeter such as disclosed in the U.S. Pat. No. to Mannherz, 4,296,636, the liquid whose flow rate is to be measured is conducted through a flow tube of a primary provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. The voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed in a secondary to actuate a recorder or indicator.
German patent publication DE-05-3132471 discloses an electromagnetic flowmeter in which the signal from the primary is applied to a secondary that includes an input amplifier and a range amplifier whose amplified output is fed to sample and hold stages to yield two half-cycle sampling or scanning values which are fed to a signal processor.
Because the electrode signal voltage from the primary includes a noise voltage and the amplifiers in the secondary produce offset voltages, unless these voltages are compensated for, the flow rate reading is inaccurate. In the German publication, a compensating voltage is derived from the output of the range amplifier, and this is added to the output voltage from the input amplifier applied to the input of the range amplifier by way of a sample and hold stage activated by a timing signal derived from the half cycle scanning values.
In this prior arrangement, the sample and hold stage for producing the compensatory voltage is activated by the timing signal once every half cycle of the signal voltage; that is to say, in each half cycle of the signal voltage a scanning takes place of the compensating voltage. Thus each half cycle scanning value is free of noise voltages as long as these are constant during one period of the signal voltage.
One significant disadvantage of this prior arrangement is that not only is a period necessary for scanning the signal, but there must also be a signal available for compensation.